Transmission line structures, such as resonators or filters, can be formed on dielectric substrates. For example, conventional stripline or microstrip resonators typically utilize a substrate which can be a ceramic or another dielectric material. For microstrip construction a metallized runner is formed on one side of the substrate with a ground plane on the other side. The stripline configuration utilizes two such structures with ground planes on the outside and the runner therebetween. A number of factors, including the dielectric constant of the substrate, the thickness of the substrate, and the length and width of the runner determine the resonant frequency of the resonator. Practical resonators and filters for high frequency, such as above 1 GHz, can be readily constructed utilizing such techniques with resonator lengths under 2 cm. It can be difficult to provide filters for lower (e.g. VHF) frequency ranges in a similar size using a similar manufacturing process. Various techniques, such as widening the capacitive end of the line or using a serpentine layout to increase the line length having been utilized to reduce the physical length required for resonance. These approaches require increased surface area of the substrate, and generally degrade the quality of the resonator.
Another approach, to resonate a transmission line filter at a lower frequency, involves the use of a lumped capacitive element, such as a chip capacitor which is connected to the transmission line structure. A disadvantage of this approach is that it is not readily tuneable. When a lumped capacitive element is not utilized, the frequency of a resonator can be readily trimmed by removing portions of the ground plane element disposed opposite to the capacitive end of the line. When a substantial portion of the capacitance is supplied by a lumped element, the removal of the ground plane has minimal effect on the capacitance, and hence, the resonant frequency of the resonator.